A recurring task in many fields of industry, particularly in the areas of production, research and development, is to examine liquids for the presence of solid substances, for example particles of powders, suspended matter, and the like.
A further task is to dissolve solid substances in the form of pastes, powders or granules in a liquid phase. One or more solid substances are measured into a container, and the liquid phase, in most cases water or an organic solvent such as ethanol, is added either by weight or by volume. The concentration of the solution resulting from the dissolution process is critically dependent on whether or not all of the solid substance components have been dissolved.
A number of different methods and systems are used to examine liquids for the presence of solid substances and to monitor dissolution processes. The simplest method is limited to a visual inspection of the liquid under examination. In the preparation of a solution, the mixture of solid substance and liquid phase in a container can be stirred until a periodic visual examination no longer reveals any visible solid components. A further method is based on prior experience as to how long a mixture of a specific solid substance and a specific solvent has to be stirred until the substance is completely dissolved.
These methods are inefficient, because the visual inspection is laborious, and if the stirring time is based on past experience, the dissolution process could take longer than necessary. As an additional problem, the visual inspection is highly susceptible to errors, since human operators are directly involved in most of the process steps.
As a way to minimize the influence of the human operator, a device for the monitoring of dissolution processes of solid substances in a liquid phase is proposed in U.S. Pat. No. 5,152,180 A, wherein the solid substance and the liquid phase are put into a container with a stirrer. The process of dissolution is monitored by sending ultrasonic signals in a wide frequency band from an emitter into the liquid phase and measuring the resonance frequency of the liquid phase by means of a detector. As more and more of the solid substance dissolves, the resonance frequency changes, and this opens the possibility to detect when the dissolution process is finished. When all of the solid substance is dissolved, the resonance frequency stops changing, and the device uses this stable state of the resonance frequency as a signal to indicate the end of the dissolution process.
The device of the foregoing description has the disadvantage that the stirrer can interfere with the measurement of the resonance frequency. Furthermore, substance particles can adhere to the wall of the container above the surface of the liquid and thus not get dissolved. Also, the emitter and the receiver are imbedded in the housing wall of the container and thus come into contact with the solution.
It is therefore the objective to propose an apparatus and a method whereby the detection of solid substances in a liquid phase is made possible or whereby the process of dissolution of a mixture of at least one solid substance and a liquid phase can be monitored without thereby contaminating the apparatus.